I will be installing an HHO system on my '02 Rio soon. I have 2 o2 sensors and was wondering if I will need to play with both of them or just the first one. The following comes from the shop manual:

"FRONT HO2 SENSOR
The front (upstream) HO2 Sensor is located in the exhaust downpipe after the TWC and provides an input voltage to the ECM. The input tells the oxygen content of the exhaust gas. The ECM uses this information to fine-tune the air/fuel ratio by adjusting injector pulse width.

REAR HO2 SENSOR
The rear (downstream) HO2 Sensor is located near the outlet end of the catalytic converter. The rear HO2 Sensor input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the rear HO2 Sensor input to the input from the front sensor, the ECM calculates catalytic convertor efficiency."

I will be installing an HHO system on my '02 Rio soon. I have 2 o2 sensors and was wondering if I will need to play with both of them or just the first one. The following comes from the shop manual:

"FRONT HO2 SENSOR
The front (upstream) HO2 Sensor is located in the exhaust downpipe after the TWC and provides an input voltage to the ECM. The input tells the oxygen content of the exhaust gas. The ECM uses this information to fine-tune the air/fuel ratio by adjusting injector pulse width.

REAR HO2 SENSOR
The rear (downstream) HO2 Sensor is located near the outlet end of the catalytic converter. The rear HO2 Sensor input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the rear HO2 Sensor input to the input from the front sensor, the ECM calculates catalytic convertor efficiency."

Planning to add HHO boost to my 07 Subaru Forester 2.5X. Got the service manual on CD and a Scangauage. I'm trying to determine what type of O2 sensor I have (wide or narrow band).

Under the hood I also have a front sensor and one in rear for the cat converter- both are 4-wire. The manual describes a "front oxygen A/F sensor" (attachments) says that with engine idling the front sensor + and - signal wires will range between 2.35V and 2.9V. But also (confusing) apparently the front O2 A/F sensor has 2 heater signal wires that range 1-14V during idle.

Think it's safe to say this is an old school narrow band sensor, controllable by EFIE?
-Mark